SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - NET LOSS PER COMMON SHARE (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted earnings per common share (in shares)	21,651	25,291	27,317
Employee stock options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted earnings per common share (in shares)	2,053	4,195	5,141
Employee stock purchase plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted earnings per common share (in shares)	25	121	153
Restricted stock and restricted stock units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted earnings per common share (in shares)	1,716	3,118	4,161
Convertible notes [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted earnings per common share (in shares)	17,857	17,857	17,862